Exhaust pipes

[n3437d]

After viewing some recent new pictures posted here I have a question needing to be answered. On the "walrus tusk" type pipes - What is the correct orientation (if any) of the openings? The pipes are usually cut at an angle and does this angled end face into the wind or downstream of the wind? Also, is the angle of cutoff critical? I have heard arguements on both sides.

[djbaker]

There is a wonderful forum topic, Exhaust Stack: Which Way Does It Go dated Mar 02/03 posted by sj. This won't help with the angle though.

[Jr.CubBuilder]

I would think that facing the angle into the wind would tend to create some backpressure in the pipe.

[dacker]

Don't think of the angle facing into the wind necessarily, but conforming to the air flow over the cowling. If you look at it that way, then you can see that the exhausts are actually perpendicular to the air flow, and that should actually create a venturi effect if anything.
I recently installed some of the Wag aero extensions, they were supposed to be two inches longer, but when I installed them they were actually the stock length. No Walrus teeth!
David

[GAHorn]

This subject was also discussed in yet another thread.
The answer can be found in the Illustrated Parts Catalog, which shows the correct orientation. The exhausts open forward and outboard.
Relative wind is insignificant in it's effect on exhaust/back-pressure performance. (Far higher pressures exist within the muffler. I.e., Can you face the wind and exhale? Then a 145 hp hot-air pump will have no difficulty at all.) The airflow of the relative wind follows the contour of the lower cowl and the forward-facing exhaust expells the gases into a lower-pressure area when faced forward/outboard. Cessna probably faced them in this way to minimise impingement onto the lower cowl. Facing them aft will pump hot exhaust gases directly toward the cowling.
Extended tailpipes add weight/moment to vibrating exhaust pipes and contribute to exhaust system cracking. (And if one were worried about the virtually unmeasurable backpressure of relative wind with forward facing pipes,....what does one suspect the increase in backpressure....not to mention additional drag posed in that relative wind....of extended tailpipes to be?)
Note: In the past such extensions had no basis of approval for installation on our aircraft (unless the airplane was operating with an experimental airworthiness certificate.) Subsequently, one mfr has obtained PMA for their pipes. (Aero-Fabricators, marketed by Wag-Aero.)
PMA normally applies to exact replacement parts. In other words, the part should conform exactly to the original (OEM) part. The "extended" pipes clearly do not meet that definition, and it's uncertain at this time whether or not they are indeed legal to install as simple replacements without further basis of approval. FYI

[N170BP]

They are PMA'd, the "man" has blessed them, good enough
for me....

Might it be that the extra 2" added to said PMA'd exhaust stacks
have been determined by the FAA to be an insignificant change
(fit/form/function). In other words, how do you "cheat" and
get the FAA to issue you a PMA for a part if it doesn't follow
basic fit/form/function rules?

I guess I have to disagree with your statement that it is uncertain
at this time whether or not they are indeed legal to install. If they're
PMA'd by the FAA, they're legal, period. If/when the PMA gets
pulled / revoked, *then* they'd be illlegal to install. If that were
to happen (admitting I'm not a regs freak), I think they'd have to
issue an AD to force you to remove said / previously installed
exhaust stacks.

I can think of numerous replacement parts that are PMA'd but
don't "strictly" conform to "exact reproduction / original" standards.
Off the cuff, one is the PMA'd gear box replacement parts you
can get from PPONK. Having drilled the originals off of and
installed the PPONK replacement parts onto an early C-180,
I can tell you that they are not "exact replicas" of the original
Cessna parts (I'm not talking about the "reinforcement/upgrade"
STC'd PPONK gear box stuff, but rather the PMA'd / replacement
gearbox parts they sell).

[GAHorn]

The reason I said it was "uncertain" is due to the response Wag Aero gave me regarding them. When asked which office had approved their PMA they couldn't say, and didn't have the document "handy".

[dkalwishky]

The reason I said it was "uncertain" is due to the response Wag Aero gave me regarding them. When asked which office had approved their PMA they couldn't say, and didn't have the document "handy".

For what it's worth I had the wag-aero pipes installed during last years annual. My shop checked into and said they would be ok/leagal to put on.

They do seem to keep the belly a little cleaner.

Dave